The invention relates to crosslinkable organopolysiloxane compositions and to a process for the preparation thereof.
Only 2,4-dichlorobenzoyl peroxide (U.S. Pat. No. 4,260,536) and para-methylbenzyl peroxide are currently suitable for crosslinking of silicone rubber extrudates under normal pressure. Both peroxides have disadvantages. Thus, 2,4-dichlorobenzoyl peroxide forms cleavage products which are in some cases toxic, and para-methylbenzyl peroxide leads to a severe odor nuisance, the rate of vulcanization is slow, and transparent vulcanization products can have a slight yellow coloration.
The object of the invention is to overcome the disadvantages of the prior art, in particular to achieve a rapid crosslinking without an odor nuisance and yellow coloration. The object is achieved by the invention, which employs specific mixtures of crosslinking agents.
The invention relates to organopolysiloxane compositions which can be prepared from an organopolysiloxane and a crosslinking agent comprising bis-4-methylbenzoyl peroxide (xe2x80x9cPMBPxe2x80x9d) and at least one compound from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (xe2x80x9cDHBPxe2x80x9d) and di-tert-butyl peroxide.
Organopolysiloxanes which can be used are all the organopolysiloxanes which can be crosslinked peroxidically, preferably organopolysiloxanes (A) of units of the general formula                               R          r                ⁢                  SiO                                    4              -              r                        2                                              (        I        )            
in which
R can be identical or different and is an optionally substituted hydrocarbon radical and
r is 0, 1, 2 or 3 and has an average numerical value of 1.9 to 2.1.
Examples of hydrocarbon radicals R are alkyl radicals such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl-, tert-butyl, n-pentyl, isopentyl, neopentyl and tert-pentyl radicals, hexyl radicals such as the n-hexyl radical, heptyl radicals such as the n-heptyl radical, octyl radicals such as the n-octyl radical and iso-octyl radicals, such as the 2,2,4-trimethylpentyl radical, nonyl radicals such as the n-nonyl radical, decyl radicals such as the n-decyl radical, dodecyl radicals such as the n-dodecyl radical, and octadecyl radicals such as the n-octadecyl radical; cycloalkyl radicals such as cyclopentyl, cyclohexyl and cycloheptyl radicals and methylcyclohexyl radicals; aryl radicals such as the phenyl, biphenyl, naphthyl and anthryl and phenanthryl radicals; alkaryl radicals, such as o-, m- and p-tolyl radicals, xylyl radicals and ethylphenyl radicals; and aralkyl radicals, such as the benzyl radical and the xcex1- and the xcex2-phenylethyl radical.
Examples of substituted hydrocarbon radicals R are halogenated alkyl radicals, such as the 3-chloropropyl, the 3,3,3-trifluoropropyl and the perfluorohexylethyl radical, and halogenated aryl radicals, such as the p-chlorophenyl and the p-chlorobenzyl radicals.
The radical R is preferably a hydrogen atom or a hydrocarbon radical having 1 to 8 carbon atoms, particularly preferably the methyl radical.
Further examples of radicals R are the vinyl, allyl, methallyl, 1-propenyl, 1-butenyl, 1-pentenyl, 5-hexenyl, butadienyl, hexadienyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, ethynyl, propargyl and 1-propynyl radicals. Preferred unsaturated radicals R are alkenyl radicals having 2 to 8 carbon atoms, particularly preferably the vinyl radical.
In the case of optionally substituted hydrocarbon radicals having 1 to 8 carbon atoms, the methyl, vinyl, phenyl and 3,3,3-trifluoropropyl radicals are particularly preferred.
Preferably, alkyl radicals, in particular methyl radicals, are bonded to at least 70 mol % of the Si atoms contained in the organopolysiloxane (A) of units of the formula (I). If the organopolysiloxanes also contain Si-bonded vinyl and/or phenyl radicals, in addition to Si-bonded methyl and/or 3,3,3-trifluoropropyl radicals, the total amount of vinyl and/or phenyl radicals is preferably 0.001-30 mol %.
The organopolysiloxanes (A) preferably predominantly comprise diorganosiloxane units. The end groups of the organopolysiloxanes can be trialkylsiloxy groups, in particular the trimethylsiloxy radical or the dimethylvinylsiloxy radical; however, one or more of these terminal alkyl groups can also be replaced by hydroxyl groups, or by alkoxy groups such as methoxy or ethoxy radicals.
The organopolysiloxanes (A) can be liquids or highly viscous substances. The organopolysiloxanes (A) preferably have a viscosity at 25xc2x0 C. of between 103 and 108 mm2/s.
The crosslinking agent used is preferably a mixture of bis-4-methylbenzoyl peroxide (PMBP) and at least one compound from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP), and di-tert-butyl peroxide in a weight ratio of 1:0.3 to 1:1, preferably in a ratio of about 1:0.8. If at least two compounds from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) and di-tert-butyl peroxide are used, these can be used in any desired mixing ratios in the context of the weight ratio first mentioned. In this case, however, mixtures of equal parts by weight of the compounds from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) and di-tert-butyl peroxide are preferred, mixtures of benzoyl peroxide and 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) in a weight ratio of 1:1 being particularly preferred.
The organopolysiloxanes (A) according to the invention furthermore preferably comprise reinforcing and/or non-reinforcing fillers. Examples of reinforcing fillers are pyrogenic or precipitated silicic acid having BET surface areas of at least 50 m2/g.
Silicic acid fillers can have a hydrophilic character or may be hydrophobicized by known processes. In this context, reference is made, for example to DE-38 39 900 A (Wacker-Chemie GmbH; filed on Nov. 25, 1988) and the corresponding U.S. Pat. No. 5,057,151. In general, the hydrophobization is then carried out with 1 to 20% by weight of hexamethyldisilazane and/or divinyltetramethyldisilizane and 0.5 to 5% by weight of water, in each case based on the total weight of the organopolysiloxane composition, these reagents advantageously being added to the organopolysiloxane (A) which have already been initially introduced into a suitable mixing device, such as, for example, a kneader or internal mixer, before the hydrophilic silicic acid is incorporated subsequently into the composition.
Examples of non-reinforcing fillers are quartz flour, diatomaceous earth, calcium silicate, zirconium silicate, zeolites, metal oxide powders, such as aluminum, titanium , iron or zinc oxide, barium silicate, barium sulfate, calcium carbonate, gypsum and powders of plastics, such as polyacrylonitrile powder or polytetrafluoroethylene powder. Fibrous components, such as glass fibers and synthetic fibers, can furthermore be employed as fillers. The BET surface area of these fillers is preferably 50 m2/g.
The organopolysiloxane compositions according to the invention, which can be crosslinked to elastomers, comprise filler (B) in amounts of preferably 1 to 200 parts by weight, particularly preferably 30 to 100 parts by weight, in each case based on 100 parts by weight of organopolysiloxane (A).
Additives (C) include processing auxiliaries such as plasticizers, pigments, and stabilizers, for example, heat stabilizers. Additives (C) can be added when necessary to the organopolysiloxane compositions vulcanizable to elastomers, depending on the particular use contemplated for the elastomer product. Non-limiting examples of plasticizers which can be employed as additives (C) include polydimethylsiloxanes which have terminal trimethylsilyl groups or hydroxyl groups and have a viscosity of not more than 1000 mm2/s at 25xc2x0 C., and diphenylsilanediol. Non-limiting examples of heat stabilizers which can be employed as additives (C) include transition metal salts of fatty acids such as iron octoate, transition metal silanolates such as iron silanolate, and cerium(IV) compounds. The compositions according to the invention moreover preferably contain no further substances apart from coloring pigments, other than those just described. The particular components employed for the preparation of the compositions according to the invention can in each case be a single component of a given type, or a mixture of at least two components of the same type.
The invention also relates to a process for the preparation of organopolysiloxane compositions which comprises mixing an organopolysiloxane and a crosslinking agent comprising bis-4-methylbenzoyl peroxide (PBMP) and at least one compound from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) and di-tert-butyl peroxide, preferably in a ratio of 1:0.3 to 1:1. The organopolysiloxane is preferably organopolysiloxane (A), which has the same meaning as above.
The organopolysiloxane compositions according to the invention which can be crosslinked peroxidically can be prepared by known processes, for example by simply mixing the individual components. The low molecular weight organosilicon compounds employed according to the invention can be incorporated here in various ways of admixing, for example in any desired step of the mixing of the individual components of the silicone rubber composition. For preparation of the compositions according to the invention which can be crosslinked peroxidically, the low molecular weight organosilicon compounds are advantageously mixed into the composition during the incorporation of the filler. Another possibility is to apply the low molecular weight organosilicon compound employed according to the invention onto the filler by mixing in a powder mixer or by mixing in a suspension of filler, inert organic solvent and low molecular weight organosilicon compound, and then stripping off the organic solvent to dryness, to be incorporated subsequently with this in the same way with the filler as a carrier. Another possibility comprises producing the low molecular weight organosilicon compound employed according to the invention on the filler by chemical reaction, for example analogously to process variant 2 described above.
The peroxidically crosslinkable compositions of the instant invention may be crosslinked under the same conditions as those described in the prior art.
The compositions according to the invention which can be crosslinked peroxidically have the advantage that the elastomers prepared therefrom show very good mechanical properties, in particular with respect to tear propagation resistance. Furthermore, the crosslinked products show no yellow coloration and do not cause an odor nuisance.
The organopolysiloxane compositions according to the invention and the elastomers prepared therefrom according to the invention can be employed for all the purposes for which organopolysiloxane compositions which can be crosslinked to elastomers or elastomers have been used to date. In particular, the organopolysiloxane compositions according to the invention and the elastomers prepared therefrom according to the invention are suitable for applications which require improved tear propagation resistance, such as, for example, hoses, cable sheathings, motor vehicle components and seals.